What is Kp index?

Photograph of Quebec, Canada taken by astronaut Donald R. Pettit, Expedition Six NASA ISS science officer, on board the International Space Station (ISS). Source: Wikipedia

In this final part of our special blog series, we will explain the origins of one of more common measurements of geomagnetic activity, the Kp index, and compare it to the solar wind power that we talk about previously.

The aurora is the most visible indicator that the solar wind or an incoming CME has caused a geomagnetic disturbance. Processes in space actually cause measurable wiggles in the whole magnetic field of Earth and can be measured on the ground. These wiggles are measured with sensitive instruments called magnetometers in units called “nano Teslas”, or nT. For over 170 years a global network of magnetometers has measured these wiggles, and there are several indices of geomagnetic activity based on them. Most of these magnetometer measurements and the subsequent indices are not measured in real-time and so will come out a few hours after the activity has arrived. During large storms, the Kp Index is the one that is most closely correlated to aurora.

Getting to know the Kp Index

The Kp index comes from the German words “planetarische kennziffer”, which translate loosely to “planetary index number” and was introduced by Julius Bartels in 1938. There are 13 magnetometer stations located around the world (from 44 – 60 degrees latitude) and they measure the level of geomagnetic fluctuation at their respective locations recorded as a “K-value” between 0 – 9 (a bit like the Richter scale used to measure earthquakes). The thing to note about the K-value is that represents the geomagnetic fluctuations for each location relative to what is historically “normal” for that area and NOT the raw value.

The K-values for each station are based off of the past 3-hours of activity at that station. The Kp index is set by averaging these 13 K-values. The map to the right shows the locations of the 13 current stations used to calculate the Kp index. A more detailed of these stations can be found here.

The relation to aurora is that a high Kp index means that there is a higher than normal geomagnetic fluctuation at the latitudes K-values are being taken at, which in turn means there is a good chance that aurora will reach those latitudes. Many aurora alerts services will let you know when Kp is predicted to increase and what level Kp might need to reach before you see the aurora at your location.

Kp Index vs Solar Wind Power

If you have explored the Aurorasaurus site, you may have noticed that we do not display the Kp index, but instead rely on solar wind power. Solar wind power is different from Kp in a few ways. First, the Kp index is determined using measurements of geomagnetic fluctuation while the solar wind power is determined using measurements of solar wind speed, magnetic field direction and strength. Second, the traditional Kp index is a measure of activity over the last 3 hours, while solar wind power is measured in real-time. A real-time estimate of the what the Kp index will be in 1 hour can be made using the same data that ACE measures and we use for solar wind power. This is called the “Wing Kp”, named for the scientist who developed the models it is based off of. More information about the Wing Kp can be found here.

For more information about the Kp index in general, you can look here. Feel free to contact us for more information about the solar wind power equation and variables!

Sean McCloat interned with Aurorasaurus in the summer of 2015 while pursuing his masters degree in Space Studies at the University of North Dakota with a focus on the planetary sciences and astrobiology. He helped analyze the project’s data, contributed to scientific papers, presentations, and blog posts, and became good friends with Rory, the Aurorasaurus plush doll mascot.